This application claims the benefit of Argentine Patent Application No. P 01 01 03353, filed on Jul. 13, 2001.
The present invention relates to a method for providing Primary Frequency Regulation in the steam turbines of combined-cycle power generation plants, such plants comprising at least one gas turbine and at least one steam turbine.
Before the development of the present invention, most power generators associated with steam turbines forming part of combined cycle plants based on gas turbines were not capable of being provided with the type of control known in the art as Primary Frequency Regulation (PFR). PFR is a function required by the agencies in charge of dispatch in power networks of many countries and is considered to be one of the parameters characterizing the quality of power delivered to customers.
The problem to be solved in steam turbines of combined cycle power plants is basically that the steam turbine is unable to supply significant amounts of additional power (between 3 and 10% of its rated capacity) within a short period of time (response time on the order of a few seconds, e.g., from 10 to 30 seconds), to effectively compensate for any variation in the mains frequency caused by sudden changes in demand. xe2x80x9cResponse timexe2x80x9d means the time it takes the turbine to reach the required final output level upon the occurrence of a step-type frequency deviation of a given magnitude.
Primary frequency regulation is not a major problem in steam turbines associated with conventional thermal cycles, that is, those where the steam used to drive the turbine is generated in a fired boiler. In these cases, the fuel-feeding loops into the boiler""s burners are adjusted to meet the required transient response, and in most cases a satisfactory performance is obtained.
On the contrary, steam turbines in combined cycles with gas turbines pose a quite different problem. In these cycles, the steam needed to drive the steam turbines comes from heat recovery boilers, which generate steam out of the heat content in gas turbine exhaust gases. In these cases, the time constants associated with the energy transfer from the exhaust gases to the water-steam circuit in the heat recovery boiler are extremely high. Therefore, upon a variation in the energy content of the exhaust gases from the gas turbines, the corresponding variations in the water-steam circuit will make them reach their final state only after several minutes. Response times of this order of magnitude are not useful to attain PFR because, as stated above, response times of several seconds are necessary.
Unlike conventional cycle steam turbines, in combined cycle steam turbines it is not possible to adjust the amount of energy fed into the boilers at will. This is because the energy fed into the heat recovery boiler comes from the exhaust gases of the gas turbine, the properties of which change according to the power output of the gas turbine. Since the gas turbine must supply Primary Frequency Regulation service, its output cannot be taken to a condition different than that required to provide said PFR service.
Consequently, steam turbines in combined cycle facilities are usually excluded from the network""s PFR, and this degrades the quality of power currently delivered by combined cycles.
Some combined cycle plants are equipped to perform an additional combustion step at some point in the gas piping connecting the gas turbine with the heat recovery boiler. This particular type of combined cycle can potentially perform a PFR function in the steam turbine by modulating fuel injection into the burners. This is a well known and valid way of providing PFR in steam turbines, which is derived from the operating procedure of steam turbines in conventional cycles, and has been widely disclosed in the related technical literature. However, the use of an additional combustion system in combined cycle plants originates in economic criteria unrelated to PFR, and only a small number of such plants make use of this system. On the other hand, due to unstable combustion as well as other technical considerations, it is not common to find combined cycle plants with additional combustion systems that provide PFR service in their steam turbines.
Sometimes combined cycle plants that are required to provide PFR service do so only with their gas turbines due to the difficulties encountered to comply with this requirement for steam turbines. Some of these plants provide xe2x80x9cover-regulationxe2x80x9d by means of their gas turbines combined, so as to make up for the deficiency of the steam turbine. However, none of these operational modes is efficient. To provide PFR with a gas turbine, it must be dispatched under its rated capacity. The reason why a turbine providing PFR must operate below its rated capacity, in normal frequency conditions, is that the turbine needs to have some extra capacity to deliver additional power when the frequency decreases. This extra capacity is called Spinning Reserve. The decrease in output in the gas turbines causes a reduction in the output of the associated steam turbine. However, such loss of power generated by the steam turbine is completely useless, because the combined cycle plant will generate less power and the managers of the system will not consider it as Spinning Reserve for PFR purposes.
Another aspect of the current state of the art is that in all combined cycle plants, irrespective of their origin, steam turbines operate normally in the xe2x80x9csliding pressurexe2x80x9d mode. In this operational mode, the steam turbine control valves remain totally open once the starting procedure of the turbine is over. Thus, all of the steam that the heat recovery boilers are capable of producing enters into the steam turbine without exerting any modulation action on the steam turbine. It is generally accepted that any throttling of the steam turbine control valves would cause a dramatic loss in output power, as well as introduce instability in the process, because the steam turbine would not be using all of the steam generated in the heat recovery boiler. This idea has frequently led combined cycle plant manufacturers and operators to think that the provision of PFR by means of steam turbines in combined cycle plants is not practical.
To solve the above-described problems, the method of the present invention makes it possible to supply PFR service to the power network by means of steam turbines operating in combined cycle plants, regardless of whether these plants are equipped with additional combustion installations in their exhaust piping. Such purpose can be achieved efficiently both from the standpoint of network frequency control, and from the standpoint of plant operation. Thus, the method of the present invention increases the quality of power delivered by combined cycle power plants.
The basic operating principle of the invention consists in storing energy in the form of internal energy of the steam contained in the dome and in the piping of the heat recovery boiler, then using the stored energy when the power demand in the network increases sharply. According to the principles of Primary Frequency Regulation, the occurrence of such an increase is marked by a decrease in frequency below its rated value. To put this basic principle to work in an efficient way, several complex controls must be performed, all of which are covered by the present method.
A further object of the present invention is to reduce to a minimum any losses in the power generated by the steam turbine, when the turbine operates in the Primary Frequency Regulation mode. As explained above, when gas turbines in a combined cycle plant operate in PFR mode, the steam turbine must be operated below its rated capacity, since gas turbines must also be operated below their rated capacity. The method of the present invention converts the decrease in the power generated by the steam turbine into a Spinning Reserve useful to implement PFR in that turbine. Thus, the Spinning Reserve is converted into readily available back-up power, available in just a few seconds. Response times as low as 10 seconds or less can be achieved if necessary. By means of the present invention, it is no longer necessary to decrease the output power of the steam turbine to a greater extent to achieve a Spinning Reserve useful for PFR.